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Southern California's asphalt streets, concrete buildings and tar-paper roofs conspire with sunlight and high temperatures to create what's known as an urban "heat island" -- an area that experiences hotter than average sweltering.

L.A . “suffers a great deal from the urban heat island effect, with impacts that include increased smog, increased heat-related illnesses and increased energy use as we try to cool our buildings,” says Noah Garrison, a lawyer with the Natural Resources Defense Council.

Advocates say the consequences of inaction are high for human health and urban sustainability – and they point to the National Climate Assessment, released Tuesday, to make the case. The assessment’s authors report that heat stress has been the leading weather-related cause of death in the country since record-keeping began. In California, a report from the state’s climate change center found that, during one particularly hot July in 2006, a 10-degree Fahrenheit increase in temperature meant a 9% higher risk of death.

Lipkis says his group wants the city to embrace a strategy for planting trees that includes adding more shades trees, native trees and food trees throughout L.A. “We want to plant the right trees in the right place to create this resilient shield to protect us from drought, from flooding, from severe heat,” he says. “There is a right mix and we’re putting it together and we’re bringing the researchers out to do that.”

Other initiatives promote changes to the built environment. Last December the city of Los Angeles changed its building code to encourage more roofs that reflect solar energy rather than absorb it. As of July 1, all new building must meet a standard for reflectivity. “And it doesn’t have to be a white roof. It doesn’t have to be ugly,” says Jonathan Parfrey, executive director of Climate Resolve, a group that pushed for the code update. “There’s like 2000 different materials that meet the standard. It could be shingle, it could be Spanish tile. They come in all varieties now.”

Garrison has advocated for more policies nationally to encourage more climate-friendly roofing and building. “Whether it’s through use of cool roofs, green roofs, or blue roofs, we need to be using more of these smart building practices that reduce the impacts development has on human health and our environment,” says Garrison, author of a study about green roofs across the country.

Garrison, Parfrey and Lipkis say this work is a good start - but just a start. Very little's been done about asphalt, for example, the biggest single contributor to urban heat. “The way it works is with asphalt-laden roads, is, they absorb all of that solar radiation, and they heat up all the neighboring buildings,” Parfrey says. He estimates around 40% of LA is covered in roadways - which means boosting reflectivity in city streets should be LA’s next big climate project.